Process for making polyester cord for no-reset v-belts



Sept. 23, 1969 R. L. KEEFE, JR 3,469,001

PROCESS FOR MAKING POLYESTER CORD FOR NO-RESET V-BELTS Filed Oct. 8, 1965 3 Sheets-Sheet 1 F I G. 1

FEED DRAW ROLL ROLL o "Wm DIP TENSION CONTROL DEVICE INVENTOR- ROBERT L. KEEFE, JR.

ATTORNEY Sept. 23, 1969 R. 1.. KEEFE, JR

PROCESS FOR MAKING POLYESTER CORD FOR NO-RESET V-BELTS 3 Sheets-Sheet 2 Filed Oct. 8, 1965 F I G. 3

3 a a i Q.

I OVEN EXIT TENSION, GRAI IS PER DENIER F I G. 4

I 2 OVEN EXIT TENSION, GRAHS PER DENIER INVENTOR ROBERT L. KEEFE,JR.

ATTORNEY Sept. 23, 1969 Filed Oct.

V- BELT TENSION AFTER IIIR., OF ORIGINAL TENSION R. L. KEEFE, JR

PROCESS FOR MAKING POLYESTER CORD FOR NO-RESET V-BEL'IS 3 Sheets-Sheet Z FIGS 11 I I I I I I I l I OVEN EXIT TENSION, GRANS PER OENIER FIG6 OVEN EXIT TENSION, GRAHS PER OENIER INVENTOR ROBERT L. KEEFE, JR.

A'TTOR NE Y 3,469,001 PROCESS FOR MAKING POLYESTER CORD FOR NQ-RESET V-BELTS Robert L. Keefe, .lr., Chadds Ford, Pa., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed Oct. 8, 1965, Ser. No. 493,976 Int. Cl. B29c 17/02 US. Cl. 264-290 4 Claims ABSTRACT OF THE DISCLOSURE The improvement in the production of V-belt reinforcing cords of polyester yarn to provide no-reset V- belts characterized by low in-use elongation which comprises passing said cord through a heating zone under a stretching tension in the range of 0.7 to 1.1 grams per denier, measured as the cord leaves the heating zone, and exposing the cord in the heating zone to a temperature between 205 and 255 C. for sufficient time to stretch the cord at least 4% net stretch based on the original unstretched length.

This invention relates to the treatment of cords composed of synthetic linear condensation polyester fibers. More particularly, it relates to a novel hot-stretching process for producing polyester cords particularly suitable for reinforcing V-belts.

Cords employed as reinforcements in elastomeric structures are frequently formed of a plurality of filaments twisted together, the twisting being provided to increase fiex strength or other desirable characteristics. Power transmission belts, such as V-belts used in automobiles, have heretofore been primarily reinforced with such cords twisted from cotton or rayon fibers. Cotton and rayon, of course, are deficient in strength and suffer from changes in length with changes in humidity. Although the newer synthetic fibers such as the polyamides and polyesters are capable of giving cords of higher strength, they have been found to suffer from dimensional instability at elevated tempertures. Polyamide cords in particular are known to elongate or grow when placed under tension and subjected to heat. Polyethylene terephthalate cords are much superior to polyamide cords in that they have a much higher modulus and exhibit a much lower tendency to grow. However, for high quality, no reset V- belts, i.e., belts which can be installed and run continuously without resetting the distance between pulleys, polyethylene terephthalate cords as heretofore prepared have not been completely satisfactory. Various hotstretching processes have been recommended to stabilize such cords but elongation under load at elevated temperatures and growth in use over a long period of time have both been excessive. Attempts to stabilize polyester cords by hot stretching at high-stretching tensions have produced cords which exhibit excessive shrinkage tensions, causing the cords to pull through the rubber during the curing of the V-belt.

The present invention provides a novel process for treating polyethylene terephthalate V-belt cords which gives results superior to those achieved by previously known processes. The cords produced have substantially the same elongation under 0.8 g.p.d. load at 100 C. temperature as they do at room temperature, and when hired States Patent 3,469,001 Patented Sept. 23, 1969 held under 0.8 g.p.d. load at C. temperature they grow only a nominal and insignificant amount. The cords produced by the process of this invention can be made into highly satisfactory V-belts showing good strength, low power loss during service, and exceptionally low growth in service.

It has now been discovered that superior polyester cords suitable for use in V-belts may be prepared by a hot-stretching process in which the cord is stretched while passing through a heating zone and the stretching tension on the cord as it leaves the heating zone is maintained within a narrow and critical tension range, as defined herein. In accordance with the invention, V-belt reinforcing cord of synthetic linear condensation polyester yarn is passed continuously through a heating zone under a stretching tension in the range of 0.7 to 1.1 grams per denier (preferably at 0.85-0.95 g.p.d.), measured as the cord leaves the heating zone, and exposing the cord in the heating zone to a temperature between 205 and 255 C. for sufiicient time to stretch the cord at least 4% based on the original unstretched length of the cord. Exposure times in the heating zone may range from about 60 seconds to about 10 minutes, but in commercial processes will be kept as short as possible for improved productivity. The exact exposure time is relatively unimportant, but the time of exposure to the temperature of 205- 255 C. must be regulated and adjusted to give the stretching tension and minimum amount of stretch specified.

Surprisingly, pretreatment of the cord before entering the stretching step described above has little effect on the fiinal cord performance. Cord fed to the abovedefined stretching step may be preheated, prestretched or prerelaxed within reasonable limits, as desired. On the other hand, it is very important that the hot-stretching step of this invention, with its critical control of stretching tension, be the last heat-treatment of the cord before it is submited to the V-belt building operation.

Although the effect of the process of this invention on internal fiber structure is not fully understood, it is found that the critical stretching conditions defined herein produce fibers having an unusually high density.

The invention will be more fully understood by reference to the specification, claims and drawings of which:

FIGURE 1 is a diagrammatic representation of a typical method of practicing the invention;

FIGURE 2 is a diagrammatic representation of a typical V-belt structure embodying the cords prepare by the present invention, in cross-section; 1

FIGURE 3 is a graph showing the change in amount of hot creep under load with change in cord stretching tension;

FIGURE 4 is a graph showing the change in cord shrinkage tension, measured at C., with change in cord stretching tension;

FIGURE 5 is a graph showing the effect of cord stretching tension on the difference between hot and cold cord elongation measured immediately after loading;

FIGURE 6 is a graph showing V-belt performance as a function of cord stretching tension.

Referring to FIGURE 2, belt 9 is composed of a tension section 10, a neutral axis section 11, and a compression section 12. The tension section 10 is composed of a plurality of layers of rubberized linear terephthalate polyester fabric bonded to compose the tension section. The neutral axis section 11 consists of a soft rubber forming a resilient cushion between the tension section 10 and the compression section 12. Reinforcing cords 13 composed of linear terephthalate polyester extend longitudinally along the belt within the neutral axis section 11. Fine fibers 14 may be dispersed uniformly through the compression section 12 to give improved strength. A layer of rubberized fabric 15 prepared from linear terephthalate polyester filaments may be placed along the bottom surface of compression section 12 for reinforcement.

In FIGURES 3 through 6, points corresponding to test values shown in Examples I to IV are labeled with the example number. The other points shows values for Samples (a) to (g) of comparison Example V and are labeled with the corresponding letters.

The process of this invention may be carried out on relatively simple equipment as, for example, a hot air oven equipped with feed rolls for supplying cord at a uniform rate, and draw rolls for withdrawing cord from the oven at a controlled higher rate determined by a device for measuring or regulating tension located between the oven and draw rolls. Suitable equipment is available commercialy but may have to be modified, in some cases, to provide an adequate degree of temperature control through the oven. Close control of temperature and a constant temperature profile along the cord from entrance to exit should be provided to produce a uniformly-treated cord with satisfactory properties.

Stretching tension may be measured by any one of a number of commercially available devices. However, it is important that the measurement be made at the exit end of the heating zone, and before the cord reaches the first draw roll. The specified tension in grams per denier is based on the initial cord denier (before stretching).

The amount of stretch to which the cord is subjected in this process, at least 4%, is given in terms of net stretch. That is, the term net stretc as used in the examples and claims refers to the amount of permanent elongation in cord length. The nominal stretch ratio of the stretching machine will usually be several percent higher that the net stretch. The amount of net stretch may be from 4% to 20% and is preferably in the range from 6% to 10%.

The temperature limits and exposure times mentioned previously may be adjusted to conform with individual machine performance and with other factors such as suitable temperatures for curing adhesive coatings and the like. Usually the lower temperatures in the indicated range will require longer heating times. Also, it will be found that cords composed of higher molecular weight polymer will require higher temperatures and longer exposure times. In any case, it should be remembered that the primary factor to be controlled in accordance with this invention is the tension on the cord during the hot-stretching step, and this must be controlled Within the critical limits indicated previously.

It is noteworthy that the presence or absence of an adhesive dip formulation on the cord as it enters the hotstretching process of this invention has no effect upon the critical cord properties. A cord with the desired properties will be obtained if the cord is stretched at least 4% with a stretching tension in the range of 0.7 to 1.1 g.p.d. in the temperature range indicated above.

The shrinkage tension referred to in the examples is the tension developed in a cord when it is heated at constant length to 160 C. The temperature 160 C. is important, as this is the normal temperature used to cure the rubber in a V-belt. The shrinkage tension value should be less than 0.35 since higher values cause the cord to pull through the rubber during the conventional belt-curing operation.

Shrinkage tension is conveniently measured on an Instron tensile testing instrument fitted with a movable oven capable of being moved into position enclosing the test sample. In the test, a single loop of test cord, relaxed 48 hours at room temperature, is suspended from the instron strain gauge hook and clamped at its lower end in the jaws of a movable clamp. Sample length is about 70 cm. The position of the movable clamp is adjusted until the strain gauge indicates a load of 0.02 g.p.d. and the clamp locked in place. Then the oven, headed to 160 C. is moved into position enclosing the sample and held there for two minutes, following which the increase in load is noted. Shrinkage tension in grams per denier is calculated by subtracting the initial load from the final load and then dividing the result by the sample denier, i.e.,

Shrinkage tension, g.p.d.=

final load, gms. initial load, gins. 2 c0rd denier The term creep, of hot creep, used in the examples refers to the cord length deformation encountered when a cord is held under load at an elevated temperature for a long period of time. Creep is measured on cord which has been allowed to relax at room temperature for at least 48 hours before testing. Two marks are made on the cord sample 50 cm. apart with the cord under a tension of about 0.02 g.p.d. All percentages are based on this length. Then the sample is heated to C. while maintaining constant length, following which the load on the cord is increased to 0.8 g.p.d. and a reading of cord length is taken at 0.01 hour. The cord is then maintained at a temperature of 100 C. for 100 hours and another length measurement is made. The percentage growth in length under load for the 100-hour period is the value used for creep in the examples. For an acceptable automotive V-belt, this creep value should not exceed about 0.6%.

The expression AE used in the examples re fers to the difference in values at 24 C. and at 100 C. of initial (after 0.01 hour) cord elongation when placed under load. AE may be used as an indication of the initial belt growth observed during belt Warm-up in service. The value AE Q is determined by measuring the percentage increase in length, at 24 C. and at 100 C.. when the loading on a 50-cm. length of cord is changed from 0.02 g.p.d. to 0.8 g.p.d. and held for 36 seconds. The 24 value is subtracted from the 100 value to give AE U For satisfactory V-belts it is preferred that the AE value be about 0, or in any case no greater than about 0.15.

The V-belt performance values in the examples refer to the percentage tension retained after 1 hour of testing under simulated use conditions as follows: In the test an automotive V-belt is installed over a drive pulley having a diameter of 6.96 inches (17.7 cm.) and two load pulleys, the first having a diameter of 2.50 inches (6.35 cm.) and the second having a diameter of 6.33 inches (16.1 cm.). The position of the pulleys is adjusted to give a belt tension of 100 lbs. and then locked in place. The drive pulley is then operated at 3,600 r.p.m. for 1 hour. Ambient temperature during this period is maintained at 71-77 C. After 1 hour the tension in the belt is measured and the test results are reported as percent of the original tension which is retained. Belts giving values in this test below 60% retained tension will require reset maintenance during use. Although this test is commonly used as an indication of performance in service, the undesirabilty of cord having a high value in the 100-hour hot-creep test may not become apparent in a one-hour performance test.

The adhesive dip used in the examples is a modified RFL adhesive mixture containing 20% solids consisting of the following three components in a 2: 1:1 weight ratio: resorcinol-formaldehyde-latex solids, the phenol adduct of methylene-bis-4-phenylisocyanate, and a commerciallyavailable insoluble polyepoxide based upon Bisphenol A which has an epoxide equivalent of 935 and a melting point of 100 C. Preparation of this adhesive is described in Krysiak US. application Ser. No. 236,153, filed Nov. 5, 1962.

The term "relative viscosity used in the examples refers to the ratio of the viscosity of a solution (10 grams per 100 ml. solution) of the polymer in a mixture of 10 parts of phenol and 7 parts of 2,4,6-trichlorophenol (by Weight) to the viscosity of the phenol-trichlorophenol mixture, per se, measured in the same units at 25 C.

EXAMPLE I Polyethylene terephthalate yarn composed of polymer having a relative viscosity of about 50 and having a denier of 1,100, a nominal tenacity of 9 g.p.d. and a break elongation of 14% is twisted and plied to an 1,100/2/5 V- belt cord with 3.5 turns per inch in the ply and 1.5 turns per inch in the cord. The cord is dipped in the adhesive mixture described previously and is then stretched by passing it through an oven at an oven temperature of 246 C. with the tension on the cord at the oven exit being 0.91 g.p.d. Exposure time is adjusted to 132 seconds to give a net stretch of 4.5%. The cord produced is found to have a shrinkage tension (160 C.) of 0.33 g.p.d., a creep value at 100 C. of 0.41% and a AE value of 0.03%.

The cord is used to build a three-eighths inch wedge (0.96 cm.-wedge) automotive V-belt of the type illustrated in FIGURE 2 having a length of 55.75 inches (141.6 cm.). The rubber in the belt is a blend of natural and neoprene rubbers. The cord is cured at 160 C. and cooled before removing from the mandrel. No cord-pullthrough is observed. The belt is then subjected to the belt performance test described previously and found to give 72% retention of tension. after 1 hour of testing. This test result indicates the superior performance of polyester cords stretched by the process of this invention.

EXAMPLE II Polyethylene terephthalate yarn composed of polymer having a relative viscosity of 31.5 and having a nominal denier of 1,100, a tenacity of 8.3 g.p.d. and break elongation of 12% is twisted and plied to an 1,100/2/5 V- belt cord with 3.6 turns per inch in the ply and 1.5 turns per inch in the cord. The cord is clipped and stretched in the same manner as that described in Example I with the net stretch being 6.8%. The cord produced is found to have a shrinkage tension at 160 C. of 0.31, a creep value at 100 C. of 0.47% and a AE1D 2 value of -0.08%.

The cord is used to build an automotive V-belt as described in Example I and tested for tension decay in the V-belt performance test, giving a value of 73% retention of tension after 1 hour of testing. This test result is considered very good in comparison with the performance of polyester and polyamide reinforced belts of the prior art.

6 EXAMPLE III This example illustrates the results obtained when the cord used in Example I is stretched with no adhesive dip applied before stretching.

Unstretched polyethylene terephthalate cord of the type used in Example I is stretched at an oven temperature of 246 C. with an applied tension on the cord of 0.91 g.p.d. Exposure time is 138 seconds and net stretch is 4.8%. The cord produced is found to have a shrinkage tension at 160 C. of 0.30 g.p.d., a creep value at C. of 0.37% and a AE L value of 0.04%. Fiber density is noted to be 1.4080 gm./cc., an unusually high value for polyethylene terephthalate fibers. These test values indicate that the cord produced is quite suitable for building a no-reset automotive V-belt.

EXAMPLE IV This example illustrates the performance of the cord used in Example 11 when no adhesive mixture is applied prior to hot stretching.

The unstretched cord described in Example II is stretched at an oven temperature of 480 C., using an applied tension of 0.87 g.p.d. Exposure time is 138 seconds and a net stretch of 5.6% is obtained. The cord produced has a shrinkage tension at C. of 0.26 g.p.d., a creep value at 100 C. of 0.34%, and a AE value of 0.0%. Fiber density is found to be 1.4056 gm./cc. As in Example III, these test values also indicate that the cord would be very satisfactory for the use in automotive V- belts.

EXAMPLE V This is a comparison example to illustrate the criticality of the process conditions defined previously. Seven cord samples are hot stretched using conditions outside the defined process limits of this invention. Each of the processed samples is unsatisfactory for the preparation of an automotive V-belt, as indicated below.

Each cord sample is of 1,100/2/5 construction, prepared from the yarn indicated in Table 1, and is stretched at the oven temperature, exposure time and tension indicated in the table. Yarn codes are explained here: Yarn sample Z, composed of fibers having a relative viscosity of 50, has a nominal tenacity of 9 g.p.d. With a break elongation of 14%. Yarn sample Y, composed of fibers having a polymer relative viscosity of 31.5, has a nominal tenacity of 8.3 g.p.d. with a break elongation of 12%. Yarn sample W is composed of polyethylene terephthalate fibers having a relative viscosity of 31.5. This yarn has a denier of 1,275 with a nominal tenacity of 6.7 g.p.d., a break elongation of 29.5%, a modulus of 80.1 g.p.d. and a shrinkage tension at 160 C. of 0.00%. Yarn X is composed of polyethylene terephthalate having a relative viscosity of 50. Yarn denier is 1,295, tenacity is 7.27 g.p.d., break elongation is 29.8%, initial modulus is 86.6 g.p.d. and shrinkage tension at 160 C. is 0.008%.

Cord properties after stretching are summarized in Table 2, along with V-belt performance data. The test values are also plotted on the accompanying graphs in FIGURES 3 through 6 for comparison with test values of the cords processed in accordance with this invention. All of the cords in Example V are unsatisfactory as shown by the fact that at least one test value falls outside of the critical limits described previously.

TABLE I.-CO RD PREPARATION TABLE IL-CORD PERFORMANCE V-Belt 160 C. Tension Shrinkage Retention Tension AE100-24 Creep (percent (g.p.d.) (percent) (percent) in 1 hr.) Remarks 0. O3 2. 9O 1. 66 30 Excessive Creep and AEmu -zw. 0. 03 3. 86 2. 29 30 D0. 0.18 0.30 0. 44 50 Excessive AE10024. 0. 14 0. 49 0. 42 53 D0. 0. 18 0. 49 0. 46 51 D0. 0. 36 0. 32 0. 85 71 Excessive shrinkage during belt cure and creep during use of belt. 0. 39 0. 38 0. 99 D0.

The present invention defines the first fully satisfactory process for preparing synthetic polyester cable cords for use in automotive V-belts. Prior cord stretching processes have been described with particular reference to tire cords and have generally specified degree of stretch and temperature of stretch. Attempts to translate tire cord stretching conditions into V-belt cord stretching conditions have been unsatisfactory, partly because of the approximately -fold increase in cord denier with the attendant problems of heat transfer throughout the cord bundle, but primarily because prior investigators have overlooked the critical nature of stretching tension in the cord stretching process. The present invention overcomes prior difiiculties by providing a process in which cords are stretched at least 4% at critical stretching tensions in the range 0.9 to 1.1 g.p.d., while allowing a reasonably wide choice in stretching temperatures and exposure times.

Although the invention has been illustrated by specific reference to cords composed of yarns made of polyethylene terephthalate, it is to be understood that the inventive concept applies equally well to other linear condensation polyester cords. Other suitable polyester yarns are described, for example in U.S. Patents Nos. 2,465,319, 2,901,466 and 2,556,295.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations.

I claim:

1. In the production of V-belt reinforcing cord of yarn of drawn fibers having a tenacity within the range of about 8 to about 12 grams per denier and an elongation at break within the range of about 8% to about 18% and consisting of synthetic linear condensation polyester having a relative viscosity of at least 30, the process for hot stretching the cord to provide substantially the same elongation at 100 C. as at 24 C. and low growth at 100 C., when tested under 0.8 gram per denier load, which comprises the final heat-treatment step of passing said cord through a heating Zone under a stretching tension in the range of 0.7 to 1.1 grams per denier, measured as the cord leaves the heating zone, and exposing the cord in the heating zone to a temperature between 205 and 255 C. for a time interval within the range of about seconds to about 10 minutes to stretch the cord from about 6 to about 10% net stretch based on the original unstretched length.

2. A process as defined in claim 1 wherein the cord is of polyethylene terephthalate and is stretched under a tension of 0.85 to 0.95 gram per denier.

3. A process as defined in claim 1 wherein the cord is stretched to provide a shrinkage tension value of less than 0.35 when measured at 160 C.

4. A process as defined in claim 1 wherein the cord is stretched to provide a value of AE L- not greater than 0.15 and a hot creep value not greater than 0.6% when measured at 100 C. under 0.8 gram per denier load per 100 hours.

References Cited UNITED STATES PATENTS 2,071,250 2/1937 Carothers 260 2,509,741 5/1950 Miles 264290 2,846,752 8/1958 Lessig 2872 2,932,078 4/1960 Wilson 28-72 3,216,187 11/1965 Chantry et al 264-290 JULIUS FROME, Primary Examiner H. MINTZ, Assistant Examiner U.S. Cl. X.R. 

